1. Field of the Invention
The present invention generally relates to radio broadcast transmission systems.
The present invention more specifically relates to a NRSC-5 transmission system incorporating a table of contents (TOC) listing program numbers for audio services and port numbers for data services.
2. Description of the Related Art
The increasingly popular High Definition (HD) Radio™ standard allows for simultaneous broadcast of digital and analog signals. HD Radio™ operates under standards adopted by the National Radio Systems Committee (NRSC). The NRSC-5 standard describes a digital radio frequency (RF) broadcast system that delivers digital audio and data services to receivers from terrestrial transmitters on existing Frequency Modulation (FM) and Amplitude Modulation (AM) radio. The NRSC-5 standard uses in-band over channel (IBOC) orthogonal frequency division multiplexing (OFDM) technology to carry digital data stream over the AM and FM bands. The system allows for coexistence of the digital modulated signal alongside the legacy analog FM and AM transmission whereby an NRSC-5 system can provide several services such multiple audio streams and data services in a single frequency band.
FIG. 1 illustrates an NRSC-5 IBOC system as is known in the art. An IBOC system can be partitioned into three sub-systems, including audio and data input subsystems; transport and service multiplex subsystem; and RF/transmission subsystem.
The audio and data subsystem describes the encapsulation of audio streams and data streams into packets that can be sent over the system. The audio streams in the IBOC system include a primary audio stream (MPS) and may include one or more secondary audio streams (SPS). The source for the primary audio stream may be a duplication of the audio stream carried over the analog signal. The source for the secondary audio channels may be new audio content not available in the analog broadcast signal.
The digital audio sources streams are compressed by the audio transport layer and sent to the transport multiplex. Compressed audio streams may be partitioned into core and enhanced packets. Decoding only the core packets results in reduced quality audio. Decoding both core and enhanced packets results in higher quality audio. The digital data can have several different stream types: program service data (PSD); station information service (SIS) data; and advanced data services (ADS).
PSD may be transmitted with the audio stream. The PSD may provide information on the audio program heard by the radio listener. PSD may include song title, artist, album, genre, comment, commercial and reference identifiers.
The SIS data may provide general information about the station programming as well as technical information. SIS may also include station identification number, station call letters, station name, station location, and station time. SIS may also include an arbitrary text message.
ADS may be carried by any broadcast data services that may carry any form and content that can be expressed as a data file or a data stream, including audio services. Examples of such services may be presentations of stock, news, weather, real time traffic and entertainment programming including audio, text and images.
A transport and service multiplex subsystem receives a multiple audio and data input stream and organizes the data stream into packets. A service multiplex arranges packets from one or more services into a logical channel (LC) stream. An IBOC system transmits one or more LC streams. Each LC stream may contain one or more audio or data services. In some cases, a single service may have some of its data transmitted over two or more LC streams.
An RF/transmission subsystem receives the LC streams and performs channel coding and waveform generation. The RF/transmission subsystem then transmits the information to an RF receiver. Each logical stream is independently coded using a convolutional encoder, then interleaved and modulated using OFDM. Each logical stream is assigned a group of tones that carries the information.
The NRSC-5 system defines several modulation profiles (MPs). Each MP is defined based on the number of OFDM sub-carriers transmitted, the number of LCs transmitted, and the bit rate that each such LC provides. FIG. 2 is a chart that lists the different MPs, the LCs enabled in each service mode and the bit-rate of each LC.
FIG. 3 is a block diagram showing components of a typical RF receiver apparatus capable of receiving communication signals transmitted over RF and decoding an NRSC-5 transmission under the current state of the art. NRSC-5-compatible receivers may be deployed in a variety of products such as car radio, portable radios, MP3 players, cellular phones and navigation devices. In a typical usage model, the user of the receiver may only be using a single service out of the several services that an NRSC-5 system provides over a single physical channel provides.
A typical radio receiver under the current state of the art is comprised of a tuner and digital signal processing hardware (DSPHW). The tuner's purpose is to tune to a desired communication channel transmitted over a specific RF band, and to down-convert the signal to an intermediate frequency (IF) or to zero frequency (DC). The down-converted signal can then be sampled by one or two analog to digital converters (ADCs) to form a digital representation of the down-converted signal.
DSPHW is implemented with digital logic circuitry in order to demodulate the sampled signal and recover the information that was modulated in the transmitted signal. The DSPHW can be hard-wired digital logic, hard programmable digital logic, programmable processor or a combination of all. The radio receiver also implements an ADC control line between the DSPHW and the ADC. The radio receiver also implements a tuner control line between the DSPHW and the tuner. The DSPHW is capable of controlling the tuner and the ADC to configure them to the desired configuration for tuning and sampling of the desired channel. The DSPHW can be capable of controlling its own power consumption by gating the clocks into some of the digital circuits of the DSPHW when they are not otherwise in use.
For example, the radio receiver may receive a tune request from a user or an application, selecting a specific service existing in a specific physical channel. The selected service may be an audio service or a data service. The current NRSC-5 protocol does not provide a TOC of available services and their allocation to LCs. The radio receiver therefore needs to receive and decode the entire stream of bits belonging to all LC's comprised in the physical channel. The radio receiver then determines what audio services and data services are comprised in each LC, and extracts from one or more LC's the appropriate decoded data corresponding to the service in use.
When decoding each logical channel, the radio receiver will need to allocate a specific amount of memory for each logical channel. This memory will be used to buffer the data stream during the steps of demodulating and decoding the bit stream, until the requested data is extracted. An example of memory buffers allocated during the decoding process may be de-interleaver buffers, which can invert the interleaving process defined in the iBiquity Digital Corporation HD Radio™ Air Interface Design Description—Layer 1 FM of Mar. 22, 2005. A second example of memory buffers is layer 1 output buffers that store the content of one or more transfer frame bits. The transfer frame and transfer frame size is defined in the aforementioned reference.
There is a need in the art for an improved NRSC-5 receiver able to take advantage of the improved protocol in order to conserve power and memory.